This invention relates to electrochemical button cells and particularly to improved leak resistant button cells and a method of assembling such cells.
Electrochemical cells of the button type are incorporated in many devices for producing electrical power. Button cells employing nonaqueous electrolytes and highly active metal anodes are particularly advantageous for certain uses while cells employing aqueous electrolytes are preferred for other uses. Some nonaqueous cells employ liquid cathodes such as sulfur dioxide or inorganic oxyhalides or thiohalides which contain dissolved metal salts, and these also serve as the electrolyte, with or without an added cosolvent. Other nonaqueous cells employ an organic solvent-based electrolyte. Aqueous cells may employ an aqueous alkaline electrolyte.
Although such cells perform remarkably well, there are certain limitations including a tendency of the electrolyte to leak after a period of time, and limited output under conditions of heavy drain service. Leakage occurs due to electrolyte creepage along the metal surface and along the gasket intended to effect a seal in such cell. Typical cell systems where leakage is a potential problem include not only the liquid cathode cells but also silver oxide-zinc cells, nickel-cadmium cells, and alkaline manganese dioxide cells.
Electrolyte leakage not only depletes the electrolyte solution from the cell but can also cause a corrosive deposit at the interface of the cover and container. This not only affects the cell appearance but can also damage the device in which the cell is used. If leakage occurs before the cell is sold, it becomes unmarketable.
Consequently, various cell structures have been proposed to limit leakage.
In some prior art button cell constructions, such as those of U.S. Pat. No. 3,185,595 and French patent No. 2,254,888, the seal gasket is constructed with a depending flange or skirt extending to the bottom of the cell container and serving to increase the electrolyte leakage path out of the cell. However, in examining the leakage performance of this type of cell construction, the inventor determined that, although the gasket is designed for a tight fit onto the container, during assembly of the cell which has the cover crimped onto the gasket, the lower portion of the gasket actually moves slightly radially inwardly away from the container wall, as the cell closing operations are performed. This weakens the seal at both the positive and negative internal interface areas, resulting in a shorter electrolyte path to the outside. Additionally, the closing operations tend to cause the corrosive electrolyte to be momentarily forced along the surfaces of the container and seal, initiating leakage paths. These factors lead to early leakage after a relatively short time period.
Referring to FIGS. 9 and 10 herein, a prior art cell is there depicted in completed assembly (FIG. 9), and enlarged (FIG. 10) to depict gasket shift away from the container wall. Various techniques in efforts to limit leakage include those in U.S. Pat. No. 4,374,909 which discloses a cell having a primary cover and a second cover, first and second gaskets, and an added outer cylindrical member over all the other elements; as well as published Japanese Application No. 59-27448 and U.S. Pat. No. 2,712,565 which set forth a cell with the container crimped around the edges of a double cover and inner gasket.